Is it 6.5 or 11mm? (.25 v. ~.48 cal.). Appears to have French proof marks, which would indicate this 11mm blackpowder German Mauser was rearesenaled to .25 cal. at the St. Denis factory in France. Is it stamped"St. Denis"?

I could not locate 'St.Denis' anywhere on the rifle...
10,95 on the receiver with the (hard to read) words Amberg?? with crown above it...I believe there is a pic on the above link.
Butt plate reads:
B
2.A.F.9
118

That rifle is too long to be a Jaeger Rifle, which has a 29 1/2 inch barrel. It is a standard infantry rifle (the "J" is a stylized "I"), made in 1883 at Amberg Arsenal, the Bavarian government arms factory. The caliber is 11mm, also called the 11mm Mauser and 11.15x60R. The Model 1871 was a Mauser design; it was a single shot, but was followed by a later rifle, the Model 71/84 which was not a conversion but a new tubular magazine repeater.

The Model 71 was a rugged and accurate rifle and was subsequently made in large quantities by Mauser, Haenel, Schilling, Spangenberg & Sauer, and by German government arsenals at Spandau, Erfurt, Amberg, and Danzig, and by Steyr in Austria. Some were even made by two companies in England for foreign sales.

The crown/L is for Ludwig II, the so-called "Mad King" of Bavaria, who was on the throne in 1883. (More info can be obtained by Googling Luwig II, Mad King, and Bavaria.)

I think ammunition or at last new cases can be had from some of the custom ammunition suppliers. It is equivalent, in both time and power to the U.S. .45-70, though the cartridges themselves are quite different.

Value? Hard to say, but if original, not cut down, and in top condition, maybe $500, but I admit not keeping up with prices on those, so you might Google the model and see what turns up on the auction sites.

If the barrel is 29 1/2", it is a Jaeger Model and I stand corrected. But there is some discrepancy, as a length of 53" should mean a barrel of 33 1/2 inches, standard rifle length. A barrel of 29 1/2 inches would give an overall length of 48 1/2".